Firstly, you need to understand the code to write smart contracts.
In a smart contract, you can specify any conditions under which certain actions will occur. For example. Ether that enters this purse is less than 1 Ether - sent back to the sender. All amounts that are greater than 1 Ether remain in the wallet and in return certain tokens are sent. Here you need to set a certain rate 1Ether = 100 tokens.
However, there are many nuances. It is necessary to provide for the withdrawal of tokens from the contract in the event of an unforeseen situation. If there is an error in the code, then additional conditions (insurance) are required.